Permeability separatory devices in which the membrane takes the form of permeable hollow fibers are now well known. Such devices offer a high membrane area per unit volume of module and do not require membrane supporting means, even when trans-membrane pressure differentials of up to 1500 pounds per square inch are employed. However, such units have not been free of support-connected problems.
It is conventional practice to pot the ends of the hollow fibers in a solid body of resin which is conveniently referred to as a tubesheet. The face of the tubesheet is sliced off (or protruding fiber loops are cut off) to permit egress of permeate (water, for example) from the fiber lumens when the fiber/tubesheet assembly is placed, together with suitable sealing means, in a pressurizeable casing and a fluid (an aqueous brine, for example) is supplied to the exterior of the unpotted fiber portions under sufficient pressure to cause permeation through the fiber walls. The pressure differential, between the back of the tubesheet -- from which the unpotted fiber portions extend -- and the tubesheet face, exerts a force which can be very substantial (as in reverse osmosis processes, for example). This force tends to deform the tubesheet and results in shear stresses which can lead to failure. Thus, some means of supporting the tubesheet is generally required, even at the expense of increased resistance to permeate egress from the fiber lumens.
Perhaps the simplest prior art tubesheet support means is a perforated metal plate as disclosed in U.S. Pat. No. 3,422,008.
An alternative support means has been disclosed (for tubesheets not located at the ends of fiber bundles) in U.S. Pat. Nos. 3,455,460 and 3,475,331. The '460 patent is directed to a type of permeator in which the hollow fibers are spirally wound, in layers, around an inner, elongated core and the tubesheet is disposed longitudinally, like a dike, rising from the core to the bundle periphery and extending from one end to the other of the bundle. The fibers passing through the tubesheet are opened by routing out a trench (into which a porous support member is inserted) or by drilling holes (which may or may not penetrate the core wall). The '331 patent discloses a spherically wound hollow fiber bundle with an equatorial tubesheet (dike) which is drilled or routed to open the fiber lumens.
Although the latter two patents issued in 1969, it does not appear to have been recognized that drilling could also be applied to end-potted bundles, thereby permitting the tubesheet to be supported simply by placing it with its face against the casing end, while avoiding any restriction of permeate egress from the fiber lumens. The only other type of support means which has been disclosed, to the best of the present applicant's knowledge, is that described in a patent (U.S. Pat. No. 3,702,658) filed after the aforesaid patents issued.
The latter patent is directed to the disposition of "porous" supports, such as particle beds, frits and screens, between the tubesheet face and the casing end. Less impedance to permeate flow, as compared to perforated plates, is said to result if certain relationships between the fiber lumen diameter and the characterizing surface dimensions of the supporting material are satisfied. However, even when an optimum match between lumen size and surface character is achieved, this type of support still, unavoidably, interferes to a substantial extent with egress of permeate from the fiber ends.